Huaxue Zhou

Common electronic origin of superconductivity in (Li,Fe)OHFeSe bulk superconductor and single-layer FeSe/SrTiO3 films

The mechanism of high-temperature superconductivity in the iron-based superconductors remains an outstanding issue in condensed matter physics. The electronic structure plays an essential role in dictating superconductivity. Recent revelation of distinct electronic structure and high-temperature superconductivity in the single-layer FeSe/SrTiO3 films provides key information on the role of Fermi surface topology and interface in inducing or enhancing superconductivity. Here we report high-resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of an FeSe-based superconductor, (Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find that this single-phase bulk superconductor shows remarkably similar electronic behaviours to that of the superconducting single-layer FeSe/SrTiO3 films in terms of Fermi surface topology, band structure and the gap symmetry. These observations provide new insights in understanding high-temperature superconductivity in the single-layer FeSe/SrTiO3 films and the mechanism of superconductivity in the bulk iron-based superconductors.

Phase Diagram of (Li1–xFex)OHFeSe: A Bridge between Iron Selenide and Arsenide Superconductors

Previous experimental results have shown important differences between iron selenide and arsenide superconductors which seem to suggest that the high-temperature superconductivity in these two subgroups of iron-based families may arise from different electronic ground states. Here we report the complete phase diagram of a newly synthesized superconducting (SC) system, (Li1-xFex)OHFeSe, with a structure similar to that of FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ∼127 K. This is the first example to demonstrate such an SDW phase in an FeSe-based superconductor system. Transmission electron microscopy shows that a well-known √5×√5 iron vacancy ordered state, resulting in an AFM order at ∼500 K in AyFe2-xSe2 (A = metal ions) superconductor systems, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q = (1)/2(1,1,0), identical to that seen in the SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.

(Li0.84Fe0.16)OHFe0.98Se superconductor: Ion-exchange synthesis of large single-crystal and highly two-dimensional electron properties

A large and high-quality single crystal (Li0.84Fe0.16)OHFe0.98Se, the optimal superconductor of a reported (Li1-xFex)OHFe1-ySe system, has been successfully synthesized via a hydrothermal ion-exchange technique. The superconducting transition temperature (T-c) of 42 K is determined by magnetic susceptibility and electric resistivity measurements, and the zero-temperature upper critical magnetic fields are evaluated as 79 and 313 T for the field along the c axis and the ab plane, respectively. The ratio of out-of-plane to in-plane electric resistivity rho(c)/rho(ab) is found to increase with decreasing temperature and to reach a high value of 2500 at 50 K, with an evident kink occurring at a characteristic temperature T* = 120 K. The negative in-plane Hall coefficient indicates that electron carriers dominate in the charge transport, and the hole contribution is significantly reduced as the temperature is lowered to approach T *. From T * down to T-c we observe the linear temperature dependencies of the in-plane electric resistivity and the magnetic susceptibility for the FeSe layers. Our findings thus reveal that the normal state of (Li0.84Fe0.16)OHFe0.98Se becomes highly two dimensional and anomalous prior to the superconducting transition, providing an insight into the mechanism of high-T-c superconductivity.

Effects R3+ on the photoluminescent properties of Ca2R8(SiO4)6O2:A?(R = Y,La,Gd;A = Eu3+,Tb3+) phosphor films prepared by the sol?gel process

Using CaCO3, metal oxides (all dissolved by nitric acid) and tetraethoxysilane Si(OC2H5)(4) (TEOS) as the main starting materials, Ca2R8(SiO4)(6)O-2:A (R = Y, La, Gd; A = EU3+, Tb3+) phosphor films have been dip-coated on quartz glass substrates through the sol-gel process. X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the 1000 degreesC annealed films are isomorphous and crystallize with the silicate oxyapatite structure. AFM and SEM studies revealed that the phosphor films consisted of homogeneous particles ranging from 30 to 90 nm, with an average thickness of 1.30 mum. The Eu3+ and Tb3+ show similar spectral properties independent of R 3, in the films due to their isomorphous crystal structures. However, both the emission intensity and lifetimes of Eu3+ and Tb3+ in Ca2R8(SiO4)(6)O-2 (R = Y, La, Gd) films decrease in the sequence of R = Gd > R = Y > R = La, which have been explained in accordance with the crystal structures.

Flux-free growth of large superconducting crystal of FeSe by traveling-solvent floating- zone technique

A flux-free solution to the growth of large and composition homogeneous superconducting FeSe crystal is reported for the first time, which is based on the traveling-solvent floating-zone technique. The size of the crystal samples prepared by this approach is up to 15x6x2 mm3, being far bigger than previously reported in all dimensions, and the main phase of the crystals is of a single preferred orientation along the tetragonal (101) plane. X-ray diffraction analysis identifies the main phase to be the superconducting tetragonal \b{eta}-FeSe. The superconducting transition temperature (TC) is determined to be 9.4 K by AC magnetic susceptibility and electronic transport measurements. A nearly perfect diamagnetic shielding of -97% is observed, indicating a bulk superconductivity in the crystal sample.

NbN superconducting nanowire single-photon detector fabricated on MgF2 substrate

The performance of superconducting nanowire single-photon detectors (SNSPDs) relies on substrate materials. Magnesium fluoride (MgF2) exhibits outstanding optical properties, such as large optical transmission range and low refractive index (n = 1.38), making it an attractive substrate. We present the fabrication and the performance of SNSPDs made of a 4.5 nm thick NbN thin film deposited on MgF2 substrate for the wavelength of 1550 nm. The front-side illuminated SNSPDs without an optical cavity showed a maximal detection efficiency of 12.8% at a system dark count rate (DCR) of 100 Hz, while the backside illuminated SNSPDs with a SiO2/Au optical cavity atop displayed a maximal detection efficiency of 33% at a DCR of 100 Hz.

Proximity-induced superconductivity within the insulating (Li0.84Fe0.16)OH layers in (Li0.84Fe0.16)OHFe0.98Se

The role played by the insulating intermediate (Li

The upper critical field and its anisotropy in (Li1−x Fe x )OHFe1−y Se

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Synthesis of large FeSe superconductor crystals via ion release/introduction and property characterization*

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Electronic Phase Separation in Iron Selenide (Li,Fe)OHFeSe Superconductor System

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